I got this error when I was running:
arcpy.FeatureClassToFeatureClass_conversion(path, gdbpath\featureDatasetA, name)
Where name matches an existing feature Class from a different dataset (i.e. gdbpath\faetureDatasetB). The really weird thing is if you were to run the tool on the correct dataset:
arcpy.FeatureClassToFeatureClass_conversion(path, gdbpath\featureDatasetB, name)
... then no errors occur and the feature class gets overwritten.
I'm running into the same behavior via ArcObjects. I want to copy a shapefile into 3 datasets within the same FGDB. The datasets are created, the first FeatureClasstoFeatureClass geoprocess goes perfect. The subsequent 2 fail with the following error message:
999999 ERROR 999999: Error executing function.
-2147467259 Failed to execute (FeatureClassToFeatureClass).